Attitude control in many spacecraft is implemented using one or more control moment gyros (CMGs). A CMG is a torque-generating mechanism that may include a rotating flywheel, or rotor, that is rotationally mounted in a gimbal frame. A spin motor may be coupled to the rotor and may be energized and controlled to rotate the rotor about a spin axis at a substantially constant rotational velocity. The gimbal frame may be rotationally mounted about one or more gimbal axes, which are perpendicular to the spin axis. Additionally, one or more actuators may be coupled to the gimbal frame and, in response to gimbal rate commands, may rotate the gimbal frame about one or more of the gimbal axes. By rotating the frame about one or more of the gimbal axes at certain rates, torques can be generated in the spacecraft body to effect spacecraft attitude control.
Although typical CMG-implemented attitude control systems, such as the one described above, are generally safe, reliable, and robust, these systems can exhibit potential drawbacks. For example, if the spacecraft is moving as a result of prior gimbal rate command having been issued to a CMG, and electrical power becomes unavailable to the CMG spin motor, or to various other components or subsystems in the spacecraft attitude control system, then the CMG, and concomitantly the spacecraft, may be subjected to undesirable mechanical stresses. These stresses may result from the rapid, uncontrolled motion of the CMG's gimbals due to gyroscopic effects from the satellite body rates.
Hence, there is a need for a system and method that mitigates or avoids undesirable mechanical stresses that can result from an unlikely loss of power to the CMG spin motor. The present invention addresses at least this need.